The present invention relates to isolating devices for transformers or other electrical equipment and more particularly to temperature and pressure responsive protective devices for use with oil-filled transformers having associated therewith automatic pressure relief valves and current limiting fuses.
In the introduction to a paper presented by three Westinghouse engineers to the 1974 Electric Utility Engineering Conference in March 1974, it was stated:
"Increasing load densities have created a trend toward larger substations, often interconnected, and shorter distribution feeders. This has led to lower distribution system impedances and a corresponding increase in the power and energy which the systems can deliver to low-impedance faults. The result has been a definite increase in the number of catastrophic failures of distribution transformers. Since these failures can be hazardous to utility personnel and to the general public, they are of increasing concern to transformer manufacturers and users alike."
The nature of such catostrophic failures was described in the following terms:
"An electric arc rapidly dissipates energy into the surrounding medium. Arcs drawn in the oil in a transformer cause almost immediate decomposition of the surrounding oil into combustible and non-condensible gases (see Table I), and propagation of a shock wave through the remaining body of oil. With sufficiently great arc power and energy, the pressure wave and its reflections will cause disruption of the transformer's enclosure. Bushings or non-welded covers may be ejected. If the cover is welded on, or otherwise rigidly secured to the tank, the tank itself may rupture. Additional arcing may then occur above the oil surface, due to breakage of leads, grounding to the tank, or exposure of energized parts by reduced oil level. Since oxygen is then present, the combustible gas may ignite and set fire to the oil spray produced by the pressure wave, causing blazing oil to be ejected violently from the disrupted tank."
The prior art for years has attempted to deal with this problem but as is apparent from the 1974 presentation the problem persists and in fact as systems become stiffer the problem is intensifying.
The prior art has suggested the use of a temperature and pressure responsive sensor for and with oil filled transformers and reference is made to U.S. Pat. No. 2,273,450 to Smith. There are two basic problems with the reference: (1) the over temperature/or pressure sensor operates a relay to control some unspecified element in the transformer circuit and (2) there appears to be no realization of benefits from the device beyond redundancy, i.e. no realization that a constant volume Boyles Law does not apply in such installations.
It has been found that the relationship between temperature and pressure in a sealed oil-filled transformer tank is not governed completely by Boyle's Law but is a variable function of the relative percentages by volume of oil and air present in the tank. The oil-filled transformer tanks, the conventional percentages of oil and air are in the order of magnitude of 90% and 10%, respectively. If the aforesaid percentages are maintained, then a pressure relief valve designed to release at 20 psig will do so at a temperature of 100.degree. C. However, either due to carelessness in initially filling the tank or in refilling the tank after maintenance or due to operation of an automatic pressure relief valve or to other factors, the above percentages may not be realized or maintained in which event the aforesaid pressure-temperature relationship is not valid.
Thus the need for the Smith device was not understood nor is the reason for its need recognized by the patentee. In addition the Smith device when placed in a transformer merely signals that a serious condition is building or actuates a relaying system and such is far from sufficient in present distribution systems.
The danger in present distribution systems is that due to short lines and high energy availability, the systems have a high fault energy availability, and in spite of the use of current limiting fuses, the amount of energy pumped into a fault before protective devices can take effect is often enough to destroy the transformer and in many cases produce an explosion. The fault energy (F) in MVA available is defined by F=KV.sup.2 /Z where Z is the effective system impedance in ohms to the point of fault where KV is the line-to-line voltage. In present systems short heavy capacity lines and large capacity generators and transformers can pump 40,000 amps into a fault, more than sufficient to produce violent situations.